1. Technical Field
The present invention relates to silicon nitride-based ceramic systems and more particularly to silicon nitride-based ceramic systems having a relatively high proportion of alpha prime SiAlON and exhibiting relatively high hardness and toughness.
2. Discussion
Silicon nitride materials currently occupy an important and growing portion of the market for materials for many commercial applications. It has become an increasingly important material, for instance, in the cutting tool and bearing industries, and continues to receive attention in the automotive component market. As compared with conventional ceramic materials, such as carbide-based materials, or steel materials (e.g., M-50), silicon nitride generally offers the potential advantages of relatively high heat resistance and chemical stability, relatively low density, good mechanical properties such as hardness and toughness, and good electrical insulation characteristics. To illustrate the advantages, in the context of the cutting tool industry, these properties can combine in whole or in part to allow operations to proceed at higher speeds and temperatures, with resulting potential cost savings. The potential for the above properties and others also makes silicon nitride an attractive material (and its uses are believed to be under exploration) for other applications, such as without limitation, extrusion dies and automotive components (e.g., without limitation, turbocharger components, swirl chambers, engine valve or valve train components, etc.).
Currently there are two widely recognized groups of silicon nitride (Si.sub.3 N.sub.4) ceramics that may be used with or even without reinforcing carbides. One group is referred to in the art as "alpha silicon nitride" (also denoted as .alpha.-Si.sub.3 N.sub.4). Until the present invention, alpha silicon nitride and its solid solutions, also called "alpha-prime SiAlON" (also denoted as .alpha.'-SiAlON; in alpha-prime SiAlON, some of the Si--N bonds in alpha silicon nitride are replaced by Al--O bonds, and the material typically includes one or more other metallic elements in addition to Si and Al), typically have exhibited relatively high hardness (e.g., on the order of about 20 GPa), but relatively low toughness (e.g., on the order about 3 MPa.multidot.m.sup.1/2). The second group is referred to in the art as "beta silicon nitride" (also denoted as .beta.-Si.sub.3 N.sub.4). Beta silicon nitride and its solid solutions, also called "beta-prime SiAlON" (denoted as .beta.'-SiAlON; in beta-prime SiAlON, some of the Si--N bonds in beta silicon nitride are replaced by Al--O bonds, but the material typically does not include other metallic elements in addition to Si and Al), typically have a hardness on the order of about 15.5 GPa, with an indentation toughness value on the order of about 6.5 MPa.multidot.m.sup.1/2. The preferred form of silicon nitride for many engineering applications has been beta silicon nitride.
In general, beta silicon nitride or beta prime SiAlON materials employ alpha silicon nitride as a starting material, and optionally may include beta-silicon nitride seeds. They are densified using any suitable conventional technique, such as pressureless sintering, gas pressure sintering, hot pressing or hot isostatic pressing. Most of the known commercial beta silicon nitride components are believed to be made by sintering, with or without a gas overpressure up to about 100 atm. The microstructure of conventional beta silicon nitride materials generally has included elongated rods of beta silicon nitride or beta prime SiAlON, which form during sintering and heat treatment, particularly where the starting materials contain a high content of alpha silicon nitride materials. These type of materials have been regarded as in-situ toughened, inasmuch as the rods are believed to form by themselves during firing and they are believed to help increase resistance to fracture in the material.
Approximate values of representative properties of beta silicon nitride-based materials are typified in the following Table I:
TABLE I ______________________________________ Density 3.3 Mg/m.sup.3 Young's Modulus 300 GPa Strength @ RT 800 Mpa Vicker Hardness/(H.sub.v) 15.5 GPa Indentation Toughness (K.sub.IC) 6.5 MPa .multidot. m.sup.1/2 Thermal Expansion 3.2 .times. 10.sup.-6 /.degree. C. ______________________________________
Some silicon nitride materials also have included reinforcing carbides to improve properties such as hardness. Previously there was not believed to be any reliable way to produce a silicon nitride material that is substantially free of carbides, but which still offers the combined advantageous properties of relatively high hardness (e.g., greater than about 19 GPa) and high toughness (e.g., greater than about 5 MPa.multidot.m.sup.1/2).
High content alpha silicon nitride (and alpha-prime SiAlON), though recognized since as early as 1978, finds considerably less application in commercial applications than does beta silicon nitride. However, powders of such materials have been used as a preferred starting material for making many beta silicon nitrides or beta prime SiAlON materials. As mentioned, the use of such starting materials has permitted the formation of elongated rods in the resulting microstructure, allowing for higher toughness. In this regard, it is believed to be well known in the art, in compositions containing (in the fired product) a combination of (1) alpha silicon nitride, alpha prime SiAlON or both, and (2) beta silicon nitride, beta prime SiAlON or both, the greater the content of the alpha or alpha prime constituent, the lower the toughness will be. Thus, the art has taught away from obtaining any-substantial amounts of alpha or alpha prime constituents in the fired material, in order to avoid the potential consequences of those phases in the final silicon nitride product.
A typical alpha prime SiAlON material has a toughness on the order of about 2.5 to 3.5 MPa.multidot.m.sup.1/2, but has a typical hardness level greater than about 19 to 21 GPa, and may even reach in some instances about 22 GPa (which tends to make it substantially higher than that of beta silicon nitride and beta prime SiAlON). It is also recognized in the art that, except under particular processing conditions (for instance, chemical vapor deposition of alpha silicon nitride (where whiskers or elongated single crystals form), or in the presence of a large amount of glass, which may degrade their strength and toughness), the attainment of elongated rod shaped grains in alpha silicon nitride and alpha prime SiAlON materials is not generally attainable after densification, such as by sintering. Because of the intrinsic difficulties in forming elongated rod shaped grains, and consequent lower toughness, alpha prime SiAlON materials have been the focus of investigation to improve toughness. The use of one or more elements such as Li, Mg, Ca, Y, Nd, Sm, Gd, Th, Dy, Ho, Er, Tm, Yb or Lu in alpha silicon nitride or alpha prime SiAlON ceramics also has been attempted, but has not yielded a commercially useful material, which combines high hardness (greater than about 19 GPa) and high toughness (greater than about 5 MPa.multidot.m.sup.1/2).
Patents to others that may be of interest include U.S. Pat. Nos. 5,468,696 (Ishizawa et al.); 5,370,716 (Mehrotra et al.); 5,316,988 (O'Brien et al.); 5,312,785 (Pyzik et al.); 5,264,297 (Jindal et al.); 5,238,885 (Asayama et al.); 5,227,346 (Hwang et al.); 5,120,687 (Hsieh); 5,120,682 (Ukyo et al.); 4,880,755 (Mehrotra et al.); 4,845,059 (Kohtoku et al.); 4,873,210 (Hsieh); 4,826,791 (Mehrotra et al.); 4,563,433 (Yeckley et al.); 4,547,470 (Tanase et al.), all of which are expressly incorporated by reference. See also, Padture, "In Situ-Toughened Silicon Carbide," J. Am. Ceram. Soc., 772!, 519-23 (1994); Mitomo et al., "Fine Grained Silicon Nitride Ceramics Prepared from .beta.-Powder," J. Am. Ceram. Soc., 781!, 211-14 (1995); Ekstrom et al., "SiAlON Ceramics," J. Am. Ceram. Soc., 752!, 259-76 (1992); Cao et al., ".alpha.'-SiAlON Ceramics: A Review," Chem. Mater. 1991, 3, 242-252; Jack, "SiAlON Ceramics: Retrospect and Prospect," Mat. Res. Soc. Symp. Proc. Vol. 287 (1993), pp. 15-27; Katz, "Applications of Silicon Nitride Based Ceramics in the U.S.," Mat. Res. Soc. Symp. Proc. Vol. 287 (1993), pp. 197-208; Hoffman, "Analysis of Microstructural Development and Mechanical Properties of Si.sub.3 N.sub.4 Ceramics," Tailoring of Mechanical Properties of Si.sub.3 N.sub.4 Ceramics (M. J. Hoffman and G. Petzow (eds.)), pp. 59-72 (1994); Shen et al., "Temperature Stability of Saemarium doped .alpha.-SiAlON Ceramics," J. Euro. Ceram. Soc., 161!, 43-53 (1996); Shen et al., "ytterbium stabilized .alpha.-SiAlON," J. Physics B: Applied Phys., 293!, 893-904 (1996); Sheu, "Microstructure and Mechanical Properties of In-situ .beta.-Si.sub.3 N.sub.4 /.alpha.'-SiAlON Composite," J. Am. Ceram. Soc., 779!, 2345-2353 (1994); all of which are expressly incorporated by reference.